Potentiometric gas sensors based on measuring the potential difference between a semiconducting metal oxide and a noble metal pseudo-reference electrode in the same gas environment offer highly selective devices that are easily manufactured and can withstand harsh environments without degrading performance. Furthermore, they are insensitive to large swings in O2 concentration, such as those that occur in a combustion exhaust. Such solid-state potentiometric gas sensors show great promise for detecting pollutants such as NOx, CO, and hydrocarbons from ppb to ppm level concentrations for exhaust monitoring. They also may be used in other applications such as in the biomedical field for breath analysis.
Potentiometric gas sensors have an output voltage signal that can be measured in many different ways and can be used to determine individual gas concentration(s) in a gas mixture or that of a varying concentration of single species in the absence of other gases. The voltage difference between two electrodes, which make up an “electrode-pair”, can be monitored as the potential at one or each electrode changes.
Potentiometric gas sensors are utilized by measuring the output voltage signal that can be used to determine individual gas concentration(s) in a gas mixture or that of a varying concentration of single species in the absence of other gases.
Solid-state potentiometric gas sensors with semiconducting metal oxide electrodes (such as p-type La2CuO4 (LCO)) have shown much promise for the monitoring of pollutant gas (such as NOx) levels in combustion exhaust. They are sensitive to ppm levels of NOx and concentrations. However, the selectivity and cross-sensitivity of these sensors is currently inadequate for commercial application. A prime example of this is the inability to discriminate between NO and NO2 (the primary components of NOx). It is often important to know the concentration of each of these individual gases; however, most NOx sensors cannot determine which of these species is present or determine their absolute concentration in mixed gas streams. In fact, poor selectivity hinders most solid-state pollutant sensors. Currently available devices for monitoring combustion exhausts and/or reaction byproducts are limited in several ways. Current devices detect only one gas species or detect multiple species only by utilizing expensive electronics to extrapolate the gas concentrations from the measurement or to take the measurement.
Current devices can require an air reference, which complicates the design, and/or have complicated manufacturing steps that increase cost.
A reference electrode is typically used to compare the changing EMF of a sensing electrode to an EMF that does not change (i.e., a reference state). A pseudo-reference is an electrode which can be used to compare all other sensing electrodes in a single gas environment. However, the pseudo-reference has an EMF that changes at the same time that the sensing electrodes are changing. Accordingly, a pseudo-reference does not actually represent a true reference state.